Fuel system for pilot burners of gasturbine engines reheat equipment



D. o. DAvlEs 5TM. 2,742,755 FUEL. SYSTEM FOR PILOT BURNERS OF GAS-TURBINE ENGINES REHEAT EQUIPMENT K 2 Sheets-Sheet l April 24, 1956 Filed Oct. 18, 1950 IOP-gw April 24, 1956 D. o. DAVIES ErAL 2,742,755

FUEL. SYSTEM FOR PILOT BuRNERs oF GAS-TURBINE ENGINES REHEAT EQUIPMENT Filed oct. 18 1950 2 sheets-sheet 2 PRESSURE FERATED sw/rcf/ pose.

United States l O i 2,742,755 l EUEL sYsTEMEoR PILOT BURNERsoF GAS- TURBINE ENGINES REHEAT EQUIPMENT v David Ornri Davies, Edgware, Charlesk Gordon Morley,

met, and since the turbine 4tiallyjconstant, the re-heat fuel system will only be in p Ashbourne, and William Ralph Coleman Ivens, Derby,

, England, assignors to Rolls-Royce Limited, Derby, England, aV British company f Application October 18, 1950, Serial No. 190,712

' Claims priority, application Great Britainl y November 14, 1949 18Claims. (Cl. 603S.6) This invention relates to fuelsystems for gas-turbine engines. l v

In some gas=turbine-eng1nes provislon is made for burn- 2,742,755 y -Patented'Apr.l 24, 1956 A,ice

inlet temperature is substanusel when the mass ow of air is substantially proportional tothe labsolute delivery pressure of the engine compressor. j

Since in preferred :arrangements according to the in vention, the fuel delivery'to the pilot burner is proportional to theabsolute delivery pressure of the compressor, it is also proportional to the'mass flow of air flowing through theengine, and therefore the fuel/air ratio of the flame'of the pilot burner is Amaintained substantially ing fuel'V in theI working tiuid lafter it has passed through one or more of the turbine stages in order to increase the power' of the engine, and additional combustion equipment supplied by a fuel systemy (hereinafter referred small proportion of the re-heat fuel flow is passed. This pilot burnermaintains a ame which stabilizes the combustion of the fuel delivered by the main burners of the system, a suitable arrangement being, for example, as described in co-pending application vSerial No. 157,732,

filed April 24, 1950, now Patent'No. 2,708,339.-

It is usual Vfor the kpilot flame to be ignited before fuel is supplied through the main burners so: as to ensure that when this main fuel supply'is injected, it is readily ignited and continues to burn.

- It will be appreciated thatfthe ame of the pilot burner should be maintained stable despite variations of the conditions in the jet pipe. In van aircraft jet propulsion engine, conditionsv in the jet pipe may vary due, for example, to changes in the altitude and forward speed of the aircraft.- An important object of the invention is to provide -arreheat fuel system supplying such a pilot burnerin a manner which results in a stable pilot yname being maintained despite such variations in conditions in the jet pipe."

According to one aspect ofl this invention, a re-heat fuel system for a gas-turbineengine, of the kind having a pilot burner and a plurality of -main burners, comprises ratio at the pilot burner.- f

According to a preferred featureof this invention,a

re-heat fuel system for aA gas-turbine engine, of the kind having a pilot burner and a plurality of main burners,

means for maintaining substantially constant the fuel/air may comprise means for maintaining the fuel lowjtov the pilot burner proportional to the absolute delivery pressure of the compressor. f

It is known that when a sonic velocity occurs-'in the l' turbine nozzles of a gas-turbine engine, the mass ow of air which passes through the engine is proportional to' the absolute delivery pressure of the compressor Vofl the engine if the turbine inlet temperatureis constant with present designs of gas-turbine engines, a sonic velocity at this point is obtained under all normal load running when an abnormally large power requirement is to be l the, ctnnpressorlf. f Y

WhenV the'..l oady to be met isV abnormal, itis usual ,to

burn fuel in the jet pipe 14, and the re-heat fuel is nor-k j the engine compressor.

constant despite variations in ambient conditions producing variations in the actual air mass ow. .This results in the llame of the pilot burner being maintained stable.

It has been vfound that when the main burners of the re-heat systemare being fed with fuel in operation of the re-heat system, .the pilot burnerrequires a greater j proportion of fuel, and laccording to al1-important feature of this invention there may be provided means whereby on initiation ofthe supply of fuel to the main fuel burners of there-heat system, the fuel/airy ratio whichvis maintainedl constant isincreased from one value to another or, in other! words, when the main burners are inoperative the fuelflow to the pilot fuel injector is maintained in one iixedproportion to `the absolute delivery pressure of the compressor and when the main burners are operative, thefuel supplyto the pilot burner is maintained in a greater proportion to lthe* absolute delivery pressure of A re-heat fuel `system which yis suitable for a gasturoine jet propulsion engine and which embodies the.v i

inventiony will now` be.- describedby way of example, referencebeing made'in vthe description to the accompanying drawings, in-which p Figure l shows diagrammatica'llya gas-turbine engine and the reheat fuel system therefor,

Figure-lAshows diagrammatically a method of electrical connection of parts illustrated in Figurel, and

`Figure .2 is an; arrangement similar to Figure'. 1 but with some parts modifed.- a

Referring to 'Figure lof,the'drawings, there is diagrammatically `shown oneform 10 of jet-propulsion gasturbine engine with which lare-heatfuel system of this invention may be employed. The engine comprises a compressor 11, main combustion'equipment 12 connected i to the delivery of the compressor 11, a turbine' 13 connec'ted to the delivery of the combustion equipment 12; i

and-an exhaust arrangement compn'sing a jet pipe 14 and propellingA nozzle 15 through which exhaust gases from the turbine pass to atmosphere. The turbine 13 drives mally supplied `through a pilotinjector device 16 comprising apair of burnersoiwnozzles 16av and a main fuel injector device f17 comprising a plurality ,of burners or nozzles 17a `lheiiarne from-the pilot injector device 16 1s employed to ignitev and ensure continued burning of the flame lfrom the"y main injector device 17.

, 4In Lvorder that the pilot injector may fullil its purpose itis 'desirable to ensure'a'stable pilot flame.

I `Voperating and when it is operating.

` The re-heatfuelfsystem'comprises two fuel pumps 20,`

. conditions, and since the re-heat system. is only. .employed y .1

The re-heat fuelsystem 'for supplying fuel to the pilot andA main injection devices 16, 17, is arranged in accordancef'with the present invention to maintain a stable pilot flame'both when the'main injector device 17 is not 21. in series, whereof thelow-pressure pump 20 is'electri cally -d'riven and isllocated=in a fuel tank 22, and whereof theghigh-p'ressurepump=21is a centrifugal pump. The f gdscharge conduit-23 of thehighfpressure pump 21v contains a shut-olf cock 24 and downstream of this cock is divided into two .branches 25, 26, whereof the branch leads to the pilot injector device 16 and whereof the branch 26 leads to a main burner fuel cock 27 and a main delivery pipe 28 leading past a throttle valve 29 to the main fuel injector device 17 of the re-heat system.

The shut-off cocks 24, 27 may be of any convenient construction and are illustrative as electrically-controlled servo-operated valves. Each comprises a disc 30 carried by a flexible diaphragm 31 separating the inlet chamber 32 'of the valve from a chamber 33 which is connected with the chamber 32 by a restriction 34 and with the outlet from the valve by a duct 35. The disc 30 engages a seat 30a around the outlet and is loaded by a spring 37 on to the seat. The outflow from chamber 33 is controlled by a lift valve 38 which is operated by a solenoid 39. When the solenoid 39 is de-energized, an outflow from chamber 33 is prevented and the disc 30 is held against its seat 30a by the spring 37; when, however, the solenoid is energized, and outflow occurs from chamber 33 through duct and the fluid pressure on the spring-loaded side of the diaphragm 31 falls so that the spring 37 is overcome by the pressure in the chamber 32 and the disc 30 lifted from its seat 30a. l

The pump 21 is driven by an air-turbine 40 which is supplied lthrough conduit 41 with air compressed by the compressor 11 of the gas-turbine engine. The air supply conduit 41 to the turbine 40 is connected to the delivery of the compressor and contains a combined control cock and throttle valve unit 42 which operates in a manner more fully described hereinafter.

The invention is applied to the above described system in order to maintain the amount of fuel supplied to the pilot fuel injector device 16 in a fixed proportion to the absolute value of the delivery pressure of the compressor 11 of the gas-turbine engine 10.

The throttle valve 43 of the unit 42 is located in the air supply conduit 41 and is connected for operation to a piston 44 which is loaded by spring 45 to bias the throttle valve 43 to the open position.' A conduit 46 leads from the discharge of the main engine fuel pump, that is the main pump of the fuel system (not shown) supplying the main combustion equipment 12, directly to the cylinder 44a of this piston 44 on one side of the piston and through a restriction 47 to the cylinder on the other side of this piston 44. The unit 42 also comprises a lift valve 48 controlling the flow through restriction 47, the valve 48 being operated by a solenoid 49. When the solenoid is de-energized the valve 48 closes off the restriction 47 so that pressure fuel from conduit 46 cannot reach the spring-loaded side of piston 44 and the latter moves to the right (as viewed in the drawing) to close throttle valve 43. When solenoid 49 is energised, pressure fluid can reach the springloaded side of the piston 44 through the restriction 47 so that the piston can act to control the extent of opening of throttle valve 43. The lift valve 48 and solenoid 49 thus act to control thethrottlevalve 43, as a shut-off cock.

When the lift-valve 4S is open, the pressure-drop across the restriction 47, and therefore across the piston 44, is controlled by a bleed valve mechanism 50 which controls a bleed flow through conduit 51 from the spring-loaded side of piston 44. The valve element 52 of the mechanism 50 is mounted on the end of a lever 53 contained in a chamber 54 which has a flexible diaphragm 55 for part of its wall. To the other side of the flexible diaphragm 55 there is a chamber 56 connected to the turbine air supply conduit 41 through a branch 41a, so that the diaphragm 55 is subjected to the delivery pressure of the engine compressor 11. The chamber 54 containing the lever 53 also contains an evacuated capsule 57 of the same effective area as the flexible diaphragm 55, and the capsule is connected to the'ilexible'diaphragm'SS by a rod 58 which is also connected to the lever 53, so that the lever 53 is loaded through th'e rod 58 with a load prol portional to the absolute delivery pressure of the engine compressor 11.

The lever 53 is carried on a perforated flexible diaphragm 53a as its fulcrum, and the chamber 54 is connected by conduit 54a to a fuel tank or the suction side of the main engine fuel pump (not shown).

The supply conduit 25 to the pilot fuel injector device 16 contains a linear flow valve mechanism 60, that is a valve mechanism across which the pressure-drop is proportional to the flow of liquid through the valve. The valve mechanism 69 comprises a body 61 having formed therein a seat 62 around an orifice the effective area of which is controlled by the head 64 of a valve element 65. The valve element 65 is loaded by a spring 66 in the sense to reduce the effective area of the orifice and the spring is so rated and the head 64 so shaped that the valve element stabilizes in a position in which the pressure drop across the orifice is proportional to the flow therethrough.

'l'he pressure-drop across this valve mechanism 60 is arranged to load a flexible diaphragm 59 forming part of the mechanism 50. The diaphragm 59 separates two chambers 59a, 59b whereof the chamber 59a is connected by a conduit 67 to the conduit 23 and the chamber 591) is connected by conduit 68 with the valve body 61 down stream of the orifice therein. The diaphragm 59 is in turn connected by a rod 69 to the lever 53 to load it in opposition to the load from the flexible diaphragm 55. Conveniently this is done by arranging (as shown) the flexible diaphragm 59 with its axis parallel to the axis of the flexible diaphragm 55, by arranging that the higher pressure acts on lthe underside of each diaphragm, and locating the fulcrum of the lever 53 between the connections to it from the two flexible diaphragms 55, 59.

The arrangement is such that an increase in the delivery pressure of compressor 11 tends to close the bleed valve 52 on to the outlet from conduit 51, which results in a reduction in pressure drop across the piston 44 operating the throttle valve 43, so that the throttle valve is opened by its spring 45 to increase the air supply to the turbine 40 of the high-pressure pump 21 and thus to increase the output of this pump so that the fuel flow to the pilot fuel `injection device 16 is increased. Increase of such fuel flow results in a higher pressure drop across the linear flow valve mechanism 60 and this results in the second flexible diaphragm 59 tending to open the bleed valve 52 and thus tending to reduce the air flow to turbine 40 and thus thespeed of the high-pres sure pump 21.

It will be appreciated that since the areas of the flexible diaphragms 55, 59 are fixed and since the distances of their connections to the lever 53 from the fulcrumforming diaphragm 53a are also fixed, the arrangement will operate to maintain the flow of fuel to the pilot fuel injector device 16 proportional to the absolute delivery pressure of the compressor 11. The flow of air through the gas-turbine engine is also proportional to the absolute delivery pressure under those conditions in which reheating is employed, and therefore the fuel/ air ratio of the pilot burner flame will be maintained constant giving a stable pilot flame.

The re-heat fuel system also comprises a branch conduit 70 fromV the supply conduit 28 of the main fuel injector device 17, downstream of the main burner fuel cock 27,` and the branch conduit 70 leads to the pilot supply conduit 25 on the downstream side of the orifice of the linear flow valve mechanism 60. This branch conduit 70 contains a second linear ow valve mechanism 71. When the main burner fuel cock 27 is opened by energising the solenoid 39 thereof, a second supply of fuel is led to the pilot fuel injector device through the second linear flow valve 71.

Since the first and second flexible diaphragms 55, 59 will continue to operate to maintain the flow through the Vlinear flow valve mechanism 60, and therefore the pressure drop across it, proportional to the absolute pump 21.

` compressor delivery pressure, and since this pressure drop is' the same as that across the second linear flow valve 71, the ow through the second linear flow valve 71 will be proportional to the absolute compressor delivery pressure. y y

The 'supply to the pilot fuel injector device. 16 will therefore still be in a fixed proportion to the absolute compressor delivery pressure although this 4fixed proportion will be greater than that in which fuel is supplied when the mainV fuel 'injector device 17 is not operative, i. e. when the main burner 'cock 27 is closed.

The fuel system also contains a pressure-operated electric switch 72 which is connected for operation to a fiexible diaphragmu'73 subjected'to the pressure' between the low-pressure vfuel" pump 20' and the high-pressure fuel This switch isr arranged to control in any convenient manner the energisation of the solenoid 49 of the combined throttle and shut-off cock unit 42 in the air supply conduitf41 to the air turbine 40 and of .the solenoids 392of the shut-off cocks 24, 27 in the delivery conduit from the high-pressure pump 21so that on'failure of the supply of fuel, the shut-off cocks are closed andthe system is put out of operation. For instance asl shown in- Figure lA, the solenoids 39, 49 may be connected to a current source through switch 72.

Thefsystem also comprises a fixed fiow restrictor 74 `in the pilot fuel delivery conduit 2S downstreamvof the linear fiow valve mechanism 60 and of the points ofthe connections of conduits 68, '70 to the delivery conduit 25. The restrictor 74 is solely for vthe purpose of facilitating adjustment of the pressure/flow Vcharacteristics of the pilot, burner 16.

Instead of only the high-pressure fuel pump 2 being driven-by an air turbine, both'the-low-pressure pump 20 and `the high-pressure pump 21 may be air turbine driven.

- Such :an arrangement isshown in Figure 2 in which those pump 2l. Thel supply branch 41a'to the turbine driving the low-pressurel pumpmay haveprovided in it an electrically controlled throttle 78 and the supply branch 41h to the `air turbine driving the high-pressure pump may haveprovided in it a throttle 43 controlled-in substantially the same manner as above'described.

'We claim:

l. vIn, a gas Vturbineengine of the kind comprising an A engine compressor, a jet pipe, and a burner in said jet pipe; means to'supply fuelto the burner comprising a fuelpump, a fuel supply conduit connecting the outlet of the fuel pump with the burner, restrictor means con nected in said conduit and comprising an orifice with a valve member to vary the restriction of the orifice which l valve member is movable by the fuel fiow through the orice in a manner to decrease the restriction and which is loaded to increase the restriction by a spring having a strength selected in relation to the effective area of the valve member to give a pressure drop in said conduit acrossthe restrictor means proportional tothe fuel flow through 'the restrictor means, a first pressure-sensitive device connected to be subjected to the absolute delivery pressureof the engine compressor, a second pressuresensitive devicel connected tobe subjected to the pressure Ydrop across .said restrictor means, and delivery-varying7 means for ycontrolling the delivery to said burner connected to be loaded in opposition by said first and second pressure-:sensitive devices. t

2. Infa gasturbine engine of the kind comprising an engine compressor, a jet pipe, and a burner in said jet pipe; means to -supply fuel tof'the burner comprising a variable-output fuel pump, a fuel supply conduit connecting the outlet of the 'fuel pump with the burner, restrictor means 'connectedin said conduit and comprising an orifice with a valve member to vary the restriction'of the orifice n which valve member is v movable by the fuel flow through the :orifice in `afmanfner Ito decrease' thev restrictionV and which Yis loadedto increase the restriction by a spring hav-ing a'r-str'eng'th selectedin relation to the effective area of the valve'member'ftofgive apressure drop in'said conduit across thev resfrict'orl means, said restrictorV means proportional to the fuel fiow through it, a rst pressuresensitive device connected to be subjectedto the absolute delivery pressure of the vengine compressor, a'second pressure-sensitive device connectedA to be subjected to the pressure drop across said restrictor 4means, and outputvarying means for said fuel pump connected to be loaded in operation by said first and 'second pressure-sensitive devices.' v

3.' In a gas turbine engine of the lkind comprising anv engine compressor, a jet pipe, a pilot burner inv said jet pipe, and a main burner in said jety pipe; means to deliver fuel to the burners comprising afuel pump, a fuel supply conduit connected with 'the outlet vof Athe fuel pump and havingrst and second branches connected respectively with the main burner and the pilot burner, restrictor means yconnected in said second branch 'and comprising an orifice with'a valve member to 'vary the restriction of the orifice which valve vmember is movable by the fuel flow through the orifice ina manner to decrease the restriction and which is loaded to increase the restriction by a spring having a strength selected in relation to the effective areav of the valv'ei'nernber to give a pressure drop across said-restrictor means proportional to the fuel ow through said restrictor means, a first pressure-sensitive device connected'to be ,subjected to the absolute delivery pressure of the engine compressor,- a second pressure-sensitive device connected to be subjected to the pressure drop across said restrictor means, and delivery-varying means for controlling the d elivery to' said 'pilot burner which delivery-varying means loaded'in opposition by said first and second pressure-sensitive devices.l

y 4. In a gas turbine engine kof the kind comprising anv engine compressor, a jet pipe, a pilot burner in said jet pipe, and a main burner in said jet pipe; and means to deliver fuel to said main burner and said pilot burner comprising a variable-output fuel pump, a fuel supply conduit connecting the outlet of the fuel pump` with the pilot burner restrictor means connected in said conduit and comprising an orifice'with alvalve member to vary the restriction of the orifice which valve member is movable by the fuel flow through the vorifice in a manner to decrease the restriction and which is loaded to increase the restriction by a spring having a strength selected in relation to the effective area of the valve memberkto give a pressure drop across said vrestrictor means proportional to the fuel flow through said "restrictormeans, a first pressure-sensitive device connected to bey subjected.y to the absolute delivery pressure of the engine compressor, a second pressure-sensitive device connected to. Vbe subjected to the pressure drop-across said restrictor'means, and output-varying means for'said fuel pump loaded in opposition by said first and second pressure-sensitive devices.

5. Inl a lgas turbine engine of the kind comprising an engine compressor', a jetpipe, a pilot burner in said jet pipe, and a main burner inv saidfjet pipe; means to deliver fuel .to the burners comprising a variable-output fuel pump, a fuel discharge conduit connected to the outlet of said pump, a first fuel supply conduit connecting said, fuel discharge conduit with the pilot burner, restrictor means connected, in said first fuel supply conduit and comprising an orifice with a valve member to vary the restriction of the orifice, which valveY member is movable by the fuel flow through the orifice in a manner to decrease the restriction and which is loadedto increase the restriction by a spring having a strength selected in relation to the effective area of the valve member to give a pressure drop across said restrictor means proportional to the fuel fiow through said restrictor means, a second fuel supply conduit connecting said fuel discharge conduit with said main burner, a branch conduit connecting said second supply conduit with first said supply conduit downstream of said restrictor means, and second restrictor means connected in said branch conduit and comprising an orifice with a valve member to vary the `restriction of the orifice which valve member is movable by the fuel flow through the orifice in a manner to decrease the restriction and which is loaded to increase the restriction by a spring having a strength selected in relation to the effective area of the valve member to give a pressure drop across said second restriction means proportional to the fuel ow through it, the pressure upstream of said second restrictor means being substantially equal to the pressure upstream of first said restrictor means when said main burner is operative, a first pressure-sensitive device connected to he subjected to the absolute delivery pressure of the engine compressor, a second pressure-,sensitive device connected to be subjected to the pressure drop across first said restrictor means, which is equal to the pressure drop across said second restrictor means when the main burner is operative, and delivery-varying means for controlling the delivery of fuel through said fuel discharge conduit which delivery-varying means is loaded in opposition by said first and second pressure-sensitive devices.

6. In a gas turbine engine of the kind comprising an engine compressor, a jet pipe, and a pilot burner in said jet pipe; means to deliver fuel to the pilot burner com prising a variable-output fuel pump, an air turbine connected to drive said fuel pump, an air supply conduit to conduct air compressed by said compressor to said-air turbine, a fuel supply conduit connecting the delivery of the fuel pump with the pilot burner, restrictor means connected in said conduit and comprising an orifice with a valve member to vary the restriction of the orifice which valve member is movable by the fuel flow through the orifice in a manner to decrease the restriction and which is loaded to increase the restriction by a spring having a strength selected in relation to the effective area of the valve member to give a pressure drop across said re strictor means proportional to the fuel flow through said restrictor means, a first pressure-sensitive device subjected to the absolute delivery pressure of the engine compressor, a second pressure-sensitive device subjected to the pressure drop across said restrictor means, and a throttle device in said air supply conduit controlled by said first and second pressure-sensitive devices acting in opposition.

7. A combination as claimed in claim 6, comprising also a source of pressure fluid, third pressure-sensitive means acted upon by said pressure fluid and operative to control said throttle device, and pressure-relieving means to relieve the pressure to which said third pressure-sensitive means is subjected and controlled by said first and second pressure-sensitive devices acting in opposition.

8. A rc-heat fuel system for a gas-turbine engine, which re-heat fuel system comprises a pilot burner, -a plurality of main burners, a variable-output fuel pump, a supply conduit connected to the delivery of the fuel pump and having first and second branches connected with the main burners and with the pilot burner, rcstrictor means connected in said second branch, which restrictor means comprises an orifice with a valve member to vary the restriction of the orifice which valve member is movable by the fuel flow through the orifice in a manner to decrease the restriction and which is loaded to increase the restriction by a spring having a strength selected in relation to the effective area of the valve member to give a pressure drop across said restrictor means proportional to the fuel flow through it, a first pressure-sensitive de vice connected to besensitive to the absolute delivery pressure of the engine compressor, a second pressure sensitive device connected to be sensitive to the pressure drop in said second branch across said restrictor means, said two pressure-sensitive devices being arranged to cooperate to control the output of the fuel pump to maintain the fuel/air ratio at the pilot burner substantially constant.

9. A re-heat fuel system as claimed in claim 8, comprising a third pressure-sensitive device arranged to control the output of said variable-output fuel pump and having said first and second pressure-sensitive devices arranged together to control the pressure to which said third pressure-sensitive device is sensitive.

10. A re-heat fuel system as claimed in claim 8, wherein said variable-output fuel pump is of the kind whereof the output is varied by varying its rotational speed and wherein said first and second pressure-sensitive devices are arranged to control the rotational speed of the pump.

1l. A re-heat fuel system as claimed in claim 10, comprising a turbine to drive said variable-output fuel pump, means to supply working fluid to said turbine, and a throttle to control the supply of working fluid to said turbine, said throttle being connected for adjustment to said first and second pressure-sensitive devices.

l2. A re-heat fuel system for a gas-turbine engine, which re-heat fuel system comprises a variable-output fuel pump, a first fuel supply conduit connected to the delivery of said fuel pump, a pilot burner connected to said first supply conduit to receive fuel therefrom, a second fuel supply conduit connected to the delivery of said fuel pump, a plurality of main burners connected to receive fuel from said second supply conduit, restrictor means connected in said first fuel supply conduit, which restrictor means comprises an orifice with a valve member to vary the restriction of the orifice which valve member is movable by the fuel ow through the orifice in a manner to decrease the restriction and which is loaded to increase the restriction by a spring having a strength selected in relation to the effective area of the valve member to give a pressure drop across said restrictor means proportional to the fuel flow through it, a first pressure-sensitive device connected to be sensitive to the absolute delivery pressure of the engine compressor, a second pressure-sensitive device connected to be sensitive to the pressure drop in said first supply conduit across said restrictor means, said two pressure-sensitive devices being arranged to co-operate to control the output of the fuel pump to maintain the fuel/air ratio at the pilot burner substantially constant, a valve means in said second supply conduit to control the fuel ow therein, and means to increase the fuel supply to the pilot burner on initiation of the supply to the main fuel burners and to maintain said increased fuel supply in a fixed proportion to the air supply at the pilot burner.

13. A re-heat fuel system as claimed in claim 12, wherein said means to increase the fuel supply to the pilot burner comprises a branch conduit connected from said second conduit at a point in said rst supply conduit to the pilot fuel burner which point is downstream of said restrictor means, and restrictor means in said branch conduit.

14. A re-heat fuel system as claimed in claim 13, wherein said restrictor means in said branch conduit is of the kind through which the fiow of fuel is proportional to the pressure drop across it.

l5. A re-heat fuel system for a gas-turbine engine, which re-heat fuel system comprises a variable-output fuel pump, a first fuel supply conduit connected to the delivery of said fuel pump, a pilot burner connected to said first supply conduit to receive fuel therefrom, a second fuel supply conduit connected to the delivery of said fuel pump, a plurality of main burners connected to receive fuel from said second supply conduit, restrictor means connected in said first fuel supply conduit, which restrictor means comprises an orifice with a valve member to vary the restriction of the orifice which valve member is movable by the fuel liow through the orice in a manner to decrease the restriction and which is loaded to increase the restriction by a spring having a strength selected in relation to the effective area of the valve member to give a pressure drop across said restrictor means proportional to the fuel ow through it, a first pressure-sensitive device comprising a first compartment divided into two cham* bers by a first flexible diaphragm and having a branch from one of the two chambers to a source of iiuid pressure and having an evacuated capsule in the other charnber connected to load the first flexible diaphragm so that the rst exible diaphragm is loaded with the resultant load proportional to the absolute uid pressure, a second pressure-sensitive device comprising a second compartment divided into two chambers by a second flexible diaphragm which has its axis parallel to the axis of the iirst diaphragm, a conduit connecting the chamber on one side of the second exible diaphragm to the upstream side of said restrictor means in the first supply conduit and a second conduit connecting the chamber on the other side of the second flexible diaphragm to the downstream side of the restrictor means, a third pressure-sensitive device arranged to control the output of said variable-output fuel pump, a lever, said iirst and second pressure-sensitive devices being connected to load said lever which carries a bleed valve determining the pressure to which the third pressure-sensitive device is subjected, the lever fulcrum, and the points at which loads are applied to the lever by the iirst and second pressure-sensitive devices being located and the effective areas of said iiexible diaphragms being selected so that increase of the fluid pressure alone results in increase in the output of the pump and so that increase of the pressure drop alone results in decrease of the output of the pump, and that the lever is balanced when the pressure-drop is in a fixed proportion to the iluid-pressure.

16. In a turbojet power plant including a compressor, and an afterburner, in combination means for automatically scheduling a predetermined air fuel mixture in said afterburner comprising a fuel pump operative to supply fuel under pressure at a variable rate to said afterburner, a pressure responsive device subject to variations in the pressure of air delivered by said compressor, an auxiliary turbine operative by air under pressure delivered by said compressor for driving said fuel pump, and valve means controlled by said pressure responsive device for varying the supply of air under pressure to the last named turbine whereby metered ow of fuel from said pump to said afterburner is insured.

17. In a gas turbine power plant having a compressor and afterburner means the combination of a fuel pump operative to supply fuel to said afterburner means, an auxiliary turbine for driving said pump operable by compressed air bled from said compressor by way of a supply communication, a valve controlling said communication, liuid pressure responsive means for measuring the mass air liow through said power plant, fuel flow measuring means responsive to the rate of supply of fuel from said fuel pump to said afterburner means, and servo mechanism controlling operation of said valve in accordance with the mass air ow and the flow of fuel from said fuel pump to said afterburner means, said mechanism including a valve element operatively connected to said tiuid pressure responsive means and also to said fuel ow measuring means.

18. In a gas turbine power plant having a compressor and afterburner means, the combination of a centrifugal fuel pump operative to supply fuel to said afterburner means, a conduit connecting said fuel pump and the afterburner means, an auxiliary turbine for driving said pump operable by compressed air bled from said compressor by Way of a supply communication, a valve controlling said communication, uid pressure operated means responsive to the pressure of air delivered by said compresser, means for establishing a pressure differential in said conduit, said pressure differential being a measure of the rate of supply of fuel from said fuel pump to said afterburner means and servomechanism controlling operation of said valve and responsive to the rate of fuel n flow from said fuel pump to said afterburner means, and

to the pressure of air delivered by said compressor, said servo-mechanism including a fluid pressure responsive valve yelement subject to the pressure differential established.

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